This invention relates to wind turbines. Wind turbines tap energy from the wind, and provide electrical energy that can be consumed locally or fed into the electrical grid. Wind energy contributes to the energy security of the United States and the rest of the world as an inexhaustible, domestic resource, thereby reducing dependence on natural gas, oil and other fossil fuels. The proposed invention will provide a means of distributed harvesting of this resource in an economically viable manner.
Wind turbines can be broadly classified based on the orientation of the axis of rotation of the rotor. The common type is the horizontal axis wind turbine (HAWT). This invention pertains to a vertical axis wind turbine (VAWT), which are attractive for suburban applications. While HAWTs are considered to have higher efficiency, they are sensitive to the direction of the wind, and also have a smaller range of wind speeds in which they can generate electric power. For domestic applications HAWTs tend to be too tall and often require expensive installation and maintenance. Previously developed VAWTs have low efficiencies, making them less attractive to suburban dwellers. Nonetheless VAWTs have some inherent advantages—they are insensitive to wind direction and require simpler installation. The proposed invention adds to the attractiveness of VAWTs by enabling higher efficiencies of energy extraction.
An early form of VAWT was the Darrieus turbine, described in U.S. Pat. No. 1,835,018 issued Dec. 8, 1931 to Darrieus. The Darrieus turbine has characteristic C-shaped blades that are connected at the top and bottom of the vertical axle. Its shape is commonly compared with that of an eggbeater. An improved version of the Darrieus turbine that incorporated upper and lower contours separated by radial stator vanes was described in U.S. Pat. No. 4,162,410 issued Jul. 24, 1979 to Amick. The Darrieus turbine is not widely used today because of its low efficiencies and structural problems.
More recently composite blades have been commonly used in VAWTs. U.S. Pat. No. 5,375,324 issued Dec. 27, 1994 to Wallace, et al. describes a pultruded composite blade for a Darrieus type wind turbine. The blade is a composite structure with a uniform cross-section with reinforcing fibers. A description of a self-erecting structure and erecting method is included.
Most VAWTs proposed in the past have fixed blades—blades that are fixed with respect to a support structure that attaches them to the main rotor. Guide vanes and deflector flaps are used to direct the wind, as described in U.S. Pat. No. 6,942,454 issued Sep. 13, 2005 to Ohlmann. There have been recent developments in creating systems that allow pivoting motion of blades, thereby enabling individual blade pitch control. U.S. Pat. No. 6,688,842 issued Feb. 10, 2004 to Boatner describes a vertical axis wind turbine having “free-flying” airfoils that self pivot according to the local dynamic conditions to which they are subjected. The motion of the airfoil about their axis is restricted to remain within limits set by stop mechanisms. The airfoil is allowed to passively—i.e., driven entirely by ambient wind conditions—pivot between a radially aligned and tangentially aligned limit. Another example of a passively controlled variable pitch vertical axis wind turbine with pitching motion constrained by stops is described in the U.S. patent application Ser. No. 11/475,459 by Jonsson.
Passive pitch control schemes generally enable better conversion of wind energy. U.S. Pat. No. 5,676,524 issued Oct. 14, 1997 to Lukas describes a VAWT having a “control plate” that can move with respect to the support structure along grooves for effecting pitch control. But allowable relative motion is restricted.
Power generation using an actively controlled blade pivoting motion can be more efficient. U.S. Pat. No. 4,247,253 issued Jan. 27, 1981 to Kazuichi Seki, et al. considers the use of active aerodynamic control for controlling the speed of a vertical axis wind turbine only for starting and braking (at excessive wind speeds) purposes. A mechanism for using spoilers for variable power control by regulated movement was presented in U.S. Pat. No. 4,500,257 issued Feb. 19, 1985 to Sullivan. U.S. Pat. No. 5,503,525 issued Apr. 2, 1996 to Brown, et al. describes a blade assembly for a vertical axis wind turbine that comprises of blades that can pivot about another vertical axis. It also describes that such a blade assembly, equipped with a wind direction measurement device and a shaft encoder, can include a control system to regulate the blade angle so that the lift component of the aerodynamic forces on the blade contributes positively to the driving torque on the rotor. However, it does not describe a specific strategy for implementing active control. U.S. Pat. No. 7,189,050 issued Mar. 13, 2007 to Taylor, et al. describes a method of increasing the efficiency of a vertical axis wind turbine through generation of a low-pressure area on a leading face of a rotor blade by using multiple stators.
Thus, there is a need for a system that improves the efficiency of power extraction capability of vertical axis wind turbines by integration of pitch, camber and generator control, and that enables efficient power extraction at any wind speed. Moreover, there is a need for a vertical axis wind turbine system that calibrates power extraction efficiency as a function of wind speed and pitch and camber control.